Electronically acquired images are usually stored in matrices comprised of rows and columns of data corresponding to the rows and columns of picture elements (pixels). The actual images as displayed on a cathode ray tube (CRT) for example, are in effect divided into rows and columns of rectangular pixels. The data in the rows and columns of the matrices corresponds to the data in each of the image rectangles.
For display purposes the values in the matrix or matrices are read out to the display monitor. In digital displays each pixel value in the matrix is transmitted to the corresponding place (by row and column) on the display monitor and the value is translated to a characteristic such as a light intensity and/or color. Many analog displays (CRTs) have a sweep mechanism that sweeps line after line. The sweep lines are made to correspond to the lines in the storage matrices. The pixel values in each line are sequentially transferred by the scanning mechanism at a rate that ensures that the translated pixel display (intensity and/or color) is spread across the image with each pixel value occupying an equal space. Thus the displayed images have M pixels displayed across each of the N lines.
The size of the memories or storage matrices are thus relatively large and represents significant components of any imaging systems. Thus any savings in the size and/or efficiency of such components will also be significant. A typical size of a matrix as defined is: EQU S=N.times.M.times.B
where:
N=the number of rows or lines PA1 M=the number of columns or pixels PA1 B=the number of bits necessary to store the largest possible pixel value.
Typical values for N, M and B in medical imaging are 512, 512 and 8 respectively; thus, each matrix comprises 2,097,152 bits. However, special applications need larger images and matrices of 33,554,152 bits have been used. For many applications a plurality of images are required per study. The relatively large size of the matrices presents severe restrictions. Presently available imaging systems either have very large on line memory capacity or the capability of storing the matrix data on an external medium (magnetic disk, for example) at a fast rate, or both.
In many imaging systems it is very important to maximize the acquisition rate. This is especially important in systems using scanning means to form the image such as for example digital X-Ray fluorography systems using TV cameras in the acquisition equipment. The usual, commercially available video cameras are limited to process data at rates up to a few megahertz (approximately 4). The scanning rate of such video cameras is synchronized with the commercially available power so that the eye will perceive smooth transitions between individual, differing images (cine mode display). Thus 50 or 60 scans per second are accomplished. A single scan therefore obviously cannot read out a 512.times.512 matrix which is the preferred size matrix with currently achievable field size and spatial resolution. In conventional prior art systems this inability to completely scan the stored data in a single scan is overcome by scanning each image in two passes. This is normally accomplished in either of the following ways:
(a) The interlace method where one pass scans the even lines, for example, and another pass scans the odd lines; and
(b) The progressive method where the first pass scans lines 1-256 and the next pass scans lines 257-512. Both methods use two passes ("fields") per image. Thus both are limited to 25 or 30 images per second. These acquisition rates impose limitations on the entire system. For example, a transfer of data to external storage means proceeds at a much slower rate. Images acquired at the full rate cannot be immediately stored.
Internal memories are relatively fast but expensive; external memories are relatively inexpensive but slow. Some presently available systems use combinations of the two types of memories using an internal memory as a "buffer", for fast storage which is subsequently "dumped" at a slower rate to an external memory. This arrangement frees the system from the constraint of the slow transfer rate for studies which are sufficiently short (the number of images is small enough) so as not to "overflow" the buffer memory. The "fast" study length is still restricted in such systems because of price considerations. An optimal system is described in the co-pending U.S. patent application entitled "Buffer Memory System", Ser. No. 487.312 filed on Apr. 21, 1983 and assigned to the assignee of this invention. Therein the dumping is done simultaneously with the acquisition.
Accordingly, scientists using imaging systems which, for example, require plurality of time separated images, are in need of means and methods for improving the imaging systems by increasing the efficiency of the image storage or memory portions of the imaging systems and also the possibility of increasing the image acquisition rate. In addition, the means and method to be described increases the "fast study" length.